Enhanced Dielectric Properties Of Graphene/P(VDF-HFP) Composites With Tailored Orientation Structure

High energy storage capacitors have been widely used in pulse power supplies,military defense,electromagnetic weapons,particle accelerators and biomedical fields.It is the main focus to improve the dielectric properties,and thus enhance the energy density and breakdown strength of dielectric material.Poly（vinylidene fluoride）（PVDF）and its copolymers are important candidates for high energy pulse capacitor.In this thesis,P（VDF-HFP）was selected as matrix to explore the influence of annealing and stretching process on crystal structure and dielectric properties.Graphene/P（VDF-HFP）composite films were prepared by solution casting to investigate the effect of graphene on the crystal structure and dielectric properties.The orientation of graphene in the composite films was studied after stretching process,and the relationship between graphene orientation and dielectric properties of composite films was also developed.The details are as following::（1）The influence of annealing temperature and heat-retained time on the crystal structure and dielectric property of P（VDF-HFP）obtained by solution casting is investigated.The effect of stretching conditions,such as stretching ratios,temperatures and rates of extension,on the relative fraction ofβ-phase and dielectric properties of P（VDF-HFP）films was investigated.The crystallinity and dielectric properties of the films were characterized by Fourier transform infrared spectroscopy（FTIR）,wide angle X-ray diffraction（WAXRD）,differential scanning calorimetry（DSC）and impedance analyzer.The fraction ofβ-phase in PVDF film increases greatly and reaches the peak of 99.7%after uniaxial stretching.Accordingly the dielectric constant of the stretched film increases by 100%and achieves up to 28.8,as well as the dielectric loss being as low as 0.007.The formula betweenβ-phase and dielectric properties was verified based on experimental data.Tuning the dielectric constant is realistic by controlling the concentration ofβ-phase in the P（VDF-HFP）film.（2）Hyperbranched polyethylene（HBPE）was performed via chain walking copolymerization at 35?C with ethylene pressure of 0.1 MPa and further applied to exfoliate graphite in CHCl₃.The graphene concentrations and structure were characterized with UV-Vis technology,high-resolution transmittance electron microscopy（HRTEM）,atom force microscopy（AFM）and Raman spectroscopy.The free-defected graphene with a concentration up to 0.97 mg/mL and less thickness（<5layers）was obtained by liquid exfoliation.The crystal phase and the crystallinity of the graphene/P（VDF-HFP）composite films were investigated by Fourier transform infrared spectroscopy（FTIR）and wide angle X-ray diffraction（WAXRD）,respectively.The transformation ofαtoβ-phase in P（VDF-HFP）was induced by the addition of graphene.The content ofβ-phase in 0.5 vol%composite film is up to87.5%with the dielectric constant of 25.1（100 Hz）and decreased surface resistivity.（3）The effect of stretching process on orientation structure and dielectric properties of graphene/P（VDF-HFP）was investigated.The kinetic equation of the relationship between the orientation angle and stretching ratio of the graphene was derived based on the Jeffery equation in uniaxial stretching flow field.The change of the content ofβphase and the crystallinity of the composite films was investigated by Fourier transform infrared spectroscopy（FTIR）and wide angle X-ray diffraction（WAXRD）.The morphology of the composite films before and after stretching was observed by AFM.The orientation angle of the graphene in the composite films with different stretching ratios was characterized by scanning electron microscopy（SEM）.The experimental orientation angles of graphene were close to the theoretical values.The dielectric properties of graphene/P（VDF-HFP）was affected significantly by deformation.When the stretching ratio was 2,the dielectric constant of the sample was 34.1（100 Hz）.With the increase of stretching ratio,the dielectric constant of composite films decreased and the dielectric loss remained steady.